Ion implanters are commonly used in the production of integrated circuits to create in a semiconductor wafer, usually silicon, regions of different conductivity by p-type or n-type doping. In such devices, a plasma source is used to ionize the dopant gas. A beam of positive ions is extracted from the source, accelerated to the desired energy, mass filtered and then directed toward the wafer. As the ions strike the wafer, they penetrate to a certain depth (depending on their kinetic energy and mass) and create regions of different electrical conductivity (depending on the dopant element concentration) into the wafer. The n-doping or p-doping nature of these regions, along with their geometrical configuration on the wafer, define their functionality, e.g., n-p-n or p-n-p junctions within the transistors. Through interconnection of many such doped regions, the wafers can be transformed into complex integrated circuits.
The amount of ion beam current is given by the rate of ion extraction from the plasma source, as shown in Equation 1:dNextr/dt≅AnsivB  (1)where A=h0×w0 is the cross-sectional area of the extraction aperture (with h0 and w0, the slit height and width, respectively), nsi the ion density at the plasma sheath edge (approximately equal to 0.61 times electron bulk density ne), and vB=(kBTe/mi)1/2 the Bohm velocity (with kB, Te and mi the Boltzmann constant, electron temperature and ion mass, respectively). Since the ion Bohm velocity for the same ionic species varies with the square root of the electron temperature, which is a slight function of plasma operating parameters, the attainable plasma density is the characteristic of interest in designing an ion source. The prior art showed that a limited number of plasma sources have proved to have sufficient plasma density to be useful as ion sources. In some embodiments, such as Bernas sources, an arc discharge creates the plasma. A flux of electrons generated by thermionic emission from tungsten filaments is used to generate and sustain the high arc plasma density. In other embodiments that use a form of arc discharge, such as indirectly heated cathodes (IHC), to reduce detrimental exposure of the filament to the plasma and therefore to extend the lifetime of the source, the necessary electrons are provided by thermionic emission from an indirectly heated cathode.
Arc based plasma sources create an acceptable amount of extracted beam current and therefore are used as ion sources on most of the present ion implanters in the semiconductor industry. However, arc based plasma sources have limited scalability. As can be seen in Equation 1, another factor that can be used for increasing the ion beam current is the cross-sectional area of the extraction slit. For a ribbon beam for which a rectangular extraction slit is used, the slit height is limited by the extraction optics, which requires narrow extraction slits for low aberration effects. Therefore the slit height is usually only a few millimeters. The slit width is limited by the availability of plasma sources to create plasmas having uniform density over large spatial dimensions. Even with the use of external magnetic fields to improve the uniformity of the plasma, arc discharge based ion sources cannot provide satisfactory (<5%) uniformity for slits wider than 90 mm. Therefore, in order to allow ion implantation of the current 300 mm diameter silicon wafer industry standard, the ion beam has to be expanded; a process that implies significant loss of beam current. For high-throughput solar cell applications or for the next generation 450 mm diameter wafer standard, wide ribbon ion beams and consequently plasma sources having good uniformity over at least 450 mm have to be developed.
One possible candidate is the inductively coupled plasma source (ICP). Unlike arc discharges, where the plasma is bounded to the arc electrodes, in this discharge, the plasma is produced by coupling the power from an RF generator to the working gas through an antenna. The high RF currents, i(t), flowing through the antenna give rise to a time varying magnetic field, B(t), as shown in Equation 2:B(t)˜i(t)  (2)which, according to the Maxwell's 3rd electrodynamics law, as shown in Equation 3:curl{right arrow over (E)}=∂{right arrow over (B)}/∂t  (3)produces intense electric fields, E, in a spatial region located in the vicinity of the antenna. Thus, electrons acquire energy from the induced electric field and are able to ionize the gas atoms and/or molecules by ionizing collisions. As the current flowing through the antenna is increased (proportional with the applied RF power), the induced electric field and implicitly the energy gained by electrons is likewise increased. Usually this power transfer from the RF source to the plasma electrons takes place within a skin depth layer in the vicinity of the RF window through ohmic (collisional) or stochastic (collisionless) heating. For collision-dominated plasmas the thickness of the layer is given by Equation 4:
                    δ        =                              (                          2                                                ωμ                  0                                ⁢                σ                                      )                                1            /            2                                              (        4        )            where ω=2πf is the RF pulsation (f is the RF frequency), μ0=4π×10−7 H/m is the magnetic permeability of vacuum, and σ, as defined by Equation 5:
                    σ        =                              ne            2                                              m              e                        ⁢                          v              c                                                          (        5        )            is the dc plasma conductivity (with n, e, me, and νc the electron density, charge, mass and collision frequency, respectively). For typical ICP plasma densities of approximately 1011 cm−3, the skin layer thickness is typically few centimeters.
Most of the ICP sources described in prior art are cylindrically shaped. FIG. 1A shows a cross-section of a prior art ICP plasma source 100. A dielectric cylinder 101 is preferably used to contain the low pressure gas and to allow RF power transmission. The cylinder is vacuum sealed at the two open ends by two metal flanges 102 and 103. For proper functioning, the gas pressure within the dielectric cylinder 101 may be maintained at less than 20 mTorr by a gas flow-gas pumping system (not shown) that employs mass flow controllers, vacuum valves, and vacuum pumps. The near flange 102 has an inlet 104 through which the desired working gas is fed into the plasma chamber 105 at a certain flow rate. The RF antenna 106 is tightly wrapped around the dielectric cylinder 101. The dielectric cylinder 101 can be comprised of any suitable material such as pyrex, quartz, or alumina. In other embodiments, a spiral shaped antenna in conjunction with a circular dielectric window and a metal cylinder are used. The RF antenna 106 is energized by an rf generator (not shown). The RF matching to the variable plasma impedance is accomplished by a matching network (not shown). The energy transferred by the RF antenna 106 to the free electrons is used to ionize the gas within the chamber 105. The distal flange 103 has a larger opening 107 to allow for vacuum pumping through the pumping port 108. A second vacuum chamber 109, electrically insulated from the flange 103 by an insulating bushing 110, contains the optics 111 used to extract the ion beam. The extraction electrodes are typically placed at the end of the cylindrical plasma chamber 105, and are aligned along a diameter of the dielectric cylinder 101.
The drawback for this geometry is that the plasma is radially non-uniform, i.e., the plasma column has a very peaked density profile on the axis of the discharge. This non-uniform plasma density profile along radial direction characteristic limits the application of this geometry for large area plasma processing. As seen in FIG. 1B, the plasma density peaks at the center of the plasma chamber 105 and decreases sharply toward the walls of the dielectric cylinder 101. For ion implantation, it follows that such a density profile can be best used for small spot-like beams, with a useful diameter of few centimeters. However, for large ion implantation throughput, a wide and high current density ribbon ion beam is desirable. Even used in conjunction with diffusion chambers to improve the radial uniformity, such plasma sources will need tremendous amount of power to create a reasonable plasma density (˜1010-1011 cm−3) across a diameter of 500-600 mm diameter.
Therefore, an ion source that can effectively utilize the relatively high plasma density produced by the ICP plasma sources but create a wide and uniform ribbon ion beam would be beneficial from ion implantation perspective.